This invention relates to a streak camera for detecting optical events occurring in ultra-short time intervals.
A streak camera has been conventionally known to detect a high-speedy optical event. In this streak camera, an optical event occurring for an ultra-short time, for example several hundreds femtoseconds, is once converted into an electron stream which is deflected in a desired direction and then the electron stream is converted to a streak image on an output screen, thereby performing a time-to-space conversion operation of the optical event. The streak camera mainly includes a streak tube comprising a photocathode for converting an incident light signal into an electron stream, a front-side acceleration means such as an acceleration electrode for accelerating the electron stream, a focusing electrode for focusing the electron stream, an anode for attracting the electron stream emitted from the photocathode, an electron deflector comprising a deflection electrode for deflecting the focused electron stream in a predetermined direction, and an electron stream detector having a phosphor screen for detecting the deflected electron stream and displaying it as a streak image thereon, these elements being arranged in this order and accommodated in a vacuum envelope, and a voltage supply unit for supplying voltages to the above elements.
As one of the conventional streak cameras, there is known a streak camera in which the anode is kept at a potential equal or lower than that of the acceleration electrode, the focusing electrode is kept at the most highly positive potential in a photocathode to-anode region, and a traveling wave deflector is used as the deflector. This type of streak camera is described in detail in "THEORETICAL AND EXPERIMENTAL STUDY OF FEMTOSECOND STREAK IMAGE TUBE" of ELECTRO-OPTICAL PRODUCTS DIVISION by H. Niu, et al. In this type of streak camera, the anode is kept at a highly-positive potential (for example, +10 KV) with respect to the photocathode in order to improve time resolution (for example, to obtain a time resolution of less than 100 femtoseconds). Accordingly, when a streak tube having an ordinary tube length is used in the streak camera, a deflection sensitivity of the streak camera using the streak tube is lowered and thus the deflection electrode of the deflector is required to be supplied with a high deflection voltage (for example, several KV voltages). This requirement causes the deflection circuit to be complicated in construction.
Further, in this type of streak camera, if a voltage difference between the photocathode and the anode is set to be a small value in order to improve the deflection sensitivity of the streak camera, an impinging electron energy of photoelectrons (defined as a kinetic energy of the photoelectrons which just impinge on the phosphor screen) is lowered and thus an signal to-noise (S/N) ratio is also lowered. Such a streak camera having a lowered S/N ratio can not be practically used.
On the other hand, there is also known another type of streak camera in which a voltage difference between the photocathode and the anode is intentionally set to a small value (for example, about 2 KV), and a rear-side acceleration means such as a mesh electrode is provided behind the deflecting electrode to increase an impinging electron energy of the photoelectrons after deflected through the deflector. However, since this type of streak camera utilizes an magnetic field to focus an electron stream emitted from the photocathode, that is, an magnetic field is used to form an electron convergent lens, the deflection sensitivity is reduced to a small value, for example, 75 mm/KV. Therefore, a high deflection voltage, for example, several kilovolts must be applied to the deflection electrode to increase the deflection sensitivity. This causes the deflection circuit to be complicated in construction like the streak camera as described above.
Generally, when a small voltage difference is provided between the photocathode and the anode to reduce a travel speed of the photoelectrons transmitted through the electron deflector, a deflection band of the deflector is equivalently lowered and thus a deflect-on voltage can not be applied to the deflection electrode at a high speed (high frequency). Accordingly, in order to perform a high-speed deflection operation, in other words, in order to supply the deflection electrode with a deflection voltage of high throughrate (V/s), a high amplitude s necessarily required for the deflection voltage.